1. Field of the Invention
The present invention relates to a ferritic stainless steel which when worked exhibits excellent atmospheric corrosion resistance and crevice corrosion resistance. Ferritic stainless steel according to the present invention is suitable to be worked and used as building exterior finish work, electric appliance parts, panels or a hot water tank bodies. Known kinds of materials heretofore were not usable for a long period of time without extensive maintenance, and required treatments for atmospheric corrosion resistance and crevice corrosion resistance.
2. Description of the Related Art
Conventional stainless steel plates, which are used as materials for building exterior finish work requiring atmospheric corrosion resistance, are used mainly in relatively small pieces, as in panels, sashes or curtain walls.
In recent years the merits of stainless steels, which merits include excellent design adaptability, fine appearance, excellent corrosion resistance and excellent atmospheric corrosion resistance, have been drawing attention. Techniques for installing such stainless steels have been developed. As a result, there is increased demand for stainless steels which can be used as large size pieces for exterior building finish work, such as roofing materials or panel materials, for example.
In that case, color painted stainless steel plates or stainless steel plates coated with a fluoroplastic have been mainly used as, for example, roofing materials.
Conventional galvanized iron roofing materials have disadvantages when painted in that the paint work tends to become faulty due to deterioration. Various types of stainless steel plates are capable of overcoming some of the disadvantages of galvanized iron materials.
Austenitic stainless steels conforming to designation SUS304 (18Cr-8Ni) have been mainly employed as stainless steel plates intended to be painted because of their excellent workability.
However, in the use of painted stainless steel plates or fluororesin coated materials, if the coating material is opaque, it is impossible to achieve a silver white metallic gloss inherent in the stainless steel. If a transparent fluororesin is used as a coating, the appearance of the surface of the stainless steel may be marred due to deterioration of the coated film. Further, since austenitic stainless steels contain a large amount of expensive Ni, they too are expensive. Also, the coefficient of thermal expansion of austenitic stainless steel is about twice that of ferritic stainless steel, and this makes austenitic stainless steel unsuited for use in elongated shapes.
Accordingly, ferritic stainless steels have recently been drawing more attention as exterior building materials.
Ferritic stainless steels, which are employed as exterior building materials, particularly as non-coated roofing materials, must exhibit excellent outdoor atmospheric corrosion resistance, even to sea salt, for a long period of time.
Where ferritic stainless steels are used as materials for exterior building finish materials, such as panels or curtain walls, since roll forming or pressing is performed in processing the stainless steel, the worked portion must also have excellent atmospheric corrosion resistance, corrosion resistance and crevice corrosion resistance.
Therefore, attempts have been made to increase the corrosion resistance of a highly atmospheric corrosion resistant and highly rust resistant ferritic stainless steel by reducing the percentages of C and N and increasing the percentages of Cr and Mo. Such a ferritic stainless steel is disclosed in, for example, Japanese Patent Laid-Open No. sho 55-138058.
However, a mere increase in the amounts of Cr and Mo produces a high alloy steel, increasing production cost and thus reducing economical usage.
Further, formability of such a steel is reduced due to hardening, and the manufacturing properties of the steel deteriorate due to its increased toughness.
Hence, there has been an increasing demand for a more inexpensive material whose atmospheric corrosion resistance, rust resistance and crevice corrosion resistance can be improved by the addition of an element other than Cr and Mo, all without formability loss of the material and loss of corrosion resistance of worked metal. It is an object of this invention to satisfy that demand. It is another object of the present invention to provide a ferritic stainless steel which is inexpensive when compared to conventional steels, and in which even worked portions such as bent or deep drawn portions exhibit excellent atmospheric corrosion resistance and crevice corrosion resistance.